The subject invention relates to a medical laser system for ablating and emulsifying biological material.
In recent years, there has been a significant amount of progress made in developing solid state laser systems for medical applications. Today, commercial systems are available which generate pulsed outputs at a variety of wavelengths. Many of the early solid state medical laser systems were based on neodymium doped gain media which generate a near infrared output wavelength of 1.06 microns. Using specific frequency selective optics, Nd:YAG medical lasers systems have also been developed which generate an output of 1.44 microns. (See, for example, U.S. Pat. No. 5,048,034).
It has been known for some time that tissue ablation can be enhanced through the use of infrared wavelengths that more closely match absorption peaks of water, the major constituent in biological tissue. To this end, the assignee herein (as well as others) has introduced medical laser systems which include a gain medium wherein holmium is the lasing species. These laser systems generate an output wavelength of 2.1 microns. The absorption coefficient in water for 2.1 micron radiation is more than two hundred times greater than the absorption coefficient of 1.06 micron radiation. This greater absorption coefficient means that more energy will be absorbed at the surface of the tissue (rather than penetrating into the tissue) resulting in cleaner ablation with less thermal damage.
One problem with the holmium laser systems is that they cannot easily be configured to generate the same output powers associated with Nd:YAG systems. To address this problem, multiple head laser systems have been developed, wherein each system includes two or more laser resonators, each having a holmium:YAG laser rod. The pulsed outputs of these laser resonators are interleaved and combined along a common output path to generate an output having a higher average power and higher repetition rate. Such system is described in U.S. Pat. No. 5,375,132, issued Dec. 20, 1994, assigned to the same assignee herein and incorporated by reference. (See also, U.S. Pat. No. 5,387,211). While the approach of using multiple resonators has been commercially successful, it is more complex and expensive than using a single resonator.
More recently, the industry has been exploring the use of erbium doped gain media for medical applications. Erbium doped YAG crystals will generate an output wavelength of 2.9 microns which is matched to a prominent absorption peak in water. Radiation at 2.9 microns has an absorption coefficient in water about two hundred times greater than 2.1 micron radiation. While this strong absorption in water would seem to make erbium laser systems the ideal candidate for tissue ablation in medical laser systems, certain problems arose.
One primary problem is related to the lack of suitable optical fibers for delivering the 2.9 micron radiation. Common silica based fibers transmit 2.9 micron radiation with very low efficiency. Fluoride based fibers, which attenuate the 2.9 micron radiation to a much lesser extent than silica fibers, are relatively toxic to the human body. More recently, improvements in the fiber delivery technology have made the use of erbium laser light more viable.
Efforts have also been made to increase the pulse repetition rate of erbium laser systems having a single resonant cavity. Quite recently, a system was introduced which could generate a pulsed output at a maximum rate of 30 Hertz. Unfortunately, attempts to increase the repetition rate, even higher resulted in a significant reduction in output power as well as an increase in the thermal loading of the laser rod leading to instabilities.
The subject invention is based, in part, on the recognition that the extremely high coefficient of absorption of 2.9 micron radiation in water allows tissue to be cleanly ablated, vaporized and/or incised at relatively low energy levels per pulse. Moreover, it has been recognized that ablation efficiency can be dramatically enhanced if a relatively high pulse repetition rate is used, even if the energy per pulse is relatively low. Optimal performance can also be enhanced by utilizing a delivery system which functions to simultaneously aspirate the tissue as it is being ablated or emulsified.
In initial experiments, the speed at which certain tissue ablation and incision procedures were performed was vastly increased using a high repetition rate, low energy per pulse, Er:YAG laser. For example, the subject laser was utilized to perform a photophacoemulsification, a common ophthalmic procedure where the lens in the eye is ablated. This procedure can take a number of minutes using a mechanical device. In contrast, using the subject erbium laser, this procedure was performed in less than 40 seconds. Other examples will be discussed below.
In accordance with the subject invention, a system is provided which includes an erbium doped, solid state gain medium mounted in a resonant cavity. The gain medium is excited by a flashlamp which is energized with a power supply. The power supply is controlled in a manner to generate a pulsed output of at least 50 hertz, and preferably greater than 100 hertz. At this repetition rate, high precision, high speed tissue ablation and smooth, fluid incisions can be achieved.
It is believed that the threshold for ablating or incising biological material can be as low as 0.5 millijoules per pulse delivered to the tissue. By optimizing the design of the laser system, it is believed that high repetition rates can be achieved while still maintaining reasonable output powers, on the order of 50 to 150 millijoules per pulse at the output coupler. At this level, the energy density needed for tissue ablation and incision can easily be delivered to the treatment site.
In one significant aspect of the subject invention, the reflectivity of the output coupler has been increased with respect to a conventionally optimized system. Increasing the reflectivity of the output coupler reduces the slope efficiency of the laser which would be a concern if the laser were to be operated at high powers. Of more importance, increasing the reflectivity of the output coupler also functions to reduce the threshold input for achieving lasing action. By this arrangement, lower input powers may be used which permit the pulse repetition rate to be increased without causing the laser to become unstable.
Another modification which can be employed to enhance the performance of the laser is to provide concave surfaces on the ends of the gain medium. These concave surfaces function to counteract the focusing effects associated with thermal lensing in the rod. It may also be desirable to include selective filters between the flashlamp and gain medium to reduce exposures to light energy which merely serves to heat the gain medium, rather than contributing to the excited states.
Additional details about the subject invention and its method of use will become apparent from the following detailed description, taken in conjunction with the drawings, in which: